Implemented mandatory options.
Merge branch 'f-mandatory-options' into develop
Behaviour outlined in test, not yet implemented.
Code simplified.
Testing if a secured file is properly loaded.
Merge branch 'f-secured-config-file' into develop
Removed badly pasted code.
Required config file existence option added.

It is now possible to require a config file to be loaded. If none is
loaded, a OSError will be raised.

Closes #6
Merge branch 'f-notfoundexception' into develop
Removed unused override.
Added a test for nonexisting & secured files.
Version bumped to 3.3.0
Documentation updated.
Merge branch 'release-3.3.0'